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Tekkaya, Ahmet Eeman
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Name Variants
Tekkaya, A. E.
Tekkaya, A. Erman
Tekkaya, AE
A.E.Tekkaya
Tekkaya,A.E.
T.,Ahmet Eeman
A.,Tekkaya
A., Tekkaya
T., Ahmet Eeman
Tekkaya, Ahmet Eeman
Ahmet Eeman, Tekkaya
Tekkaya, E
Tekkaya, E. A.
Tekkaya, A. Erman
Tekkaya, AE
A.E.Tekkaya
Tekkaya,A.E.
T.,Ahmet Eeman
A.,Tekkaya
A., Tekkaya
T., Ahmet Eeman
Tekkaya, Ahmet Eeman
Ahmet Eeman, Tekkaya
Tekkaya, E
Tekkaya, E. A.
Job Title
Profesör Doktor
Email Address
Main Affiliation
Manufacturing Engineering
Status
Former Staff
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Scholarly Output
31
Articles
13
Citation Count
867
Supervised Theses
2
31 results
Scholarly Output Search Results
Now showing 1 - 10 of 31
Review Citation - WoS: 54Citation - Scopus: 58The Development of Ring Rolling Technology - Part 2: Investigation of Process Behaviour and Production Equipment(Wiley-blackwell, 2005) Allwood, JM; Tekkaya, AE; Stanistreet, TF; Manufacturing EngineeringThis is the continuation of a thorough survey of work on ring rolling published in the English and German languages up to 2004 [see No. 2/3, p. 111-120]: The methods used to investigate the process are reviewed, separated into experimental and theoretical categories; the insights gained from these investigations are organised according to the challenges identified at the outset; developments in the control and operation of the process are described. Having given a set of ideal targets for the process, the state of current knowledge about ring rolling is assessed in order to predict likely developments: process modelling capability is nearly able to predict rolling behaviour for a complete cycle with sufficient accuracy to allow effective use of models for design of rolling schedules and preforms; analysis of material behaviour is relatively mature for steel rings, but has scope for significant extension for titanium and aluminium alloys and composites; design choices that seek to extend the flexibility of the process have had some exploration, but could be extended.Article Citation - WoS: 54Citation - Scopus: 62Residual stress state and hardness depth in electric discharge machining: De-ionized water as dielectric liquid(Taylor & Francis inc, 2005) Ekmekci, B; Elkoca, O; Tekkaya, AE; Erden, A; Manufacturing Engineering; Department of Mechatronics EngineeringProcedures and results of experimental work to measure residual stresses and hardness depth in electric discharge machined surfaces are presented. Layer removal method is used to express the residual stress profile as a function of depth caused by a die sinking type EDM. Thin stressed layers are removed from machined samples by electrochemical machining. Corresponding deformations due to stress relaxation are recorded for each removal to determine the stress profile from elasticity theory. The relational dependence of the machining parameters with residual stresses is obtained and a semi-empirical model is proposed for plastic mold steel for de-ionized water as dielectric liquid. These stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value, around the yield strength, and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses.Conference Object Citation - Scopus: 0Analysis of Force-Indentation Measurements on Anisotropic Metals(Edizioni libreria progetto, 2005) Koray,E.; Topcu,N.; Tekkaya,A.E.; Manufacturing EngineeringIndentation tests are widely used with simultaneous measurements of indentation depth and force especially for determining material properties. In this study, findings on parameters affecting the force-displacement curves obtained from indentations on anisotropic metals using special not self-similar indenters will be presented. Parameters such as specimen dimensions, friction, surface roughness, specimen clamping and indentation spacing have been investigated by finite element analysis and by experiments. Optimum values of these parameters to achieve higher repeatability and accuracy are determined.Article Citation - WoS: 5Citation - Scopus: 6Material Flow Control in High Pressure Sheet Metal Forming of Large Area Parts With Complex Geometry Details(verlag Stahleisen Mbh, 2005) Trompeter, M; Önder, E; Homberg, W; Tekkaya, E; Kleiner, M; Manufacturing EngineeringWorking media based forming processes show advantages compared to the conventional deep drawing in the range of sheet metal parts with complex geometry details. By High Pressure Sheet Metal Forming (HBU), complex parts can be formed with reduced tool costs, fewer process steps, and improved part properties, particularly by the use of high strength steels. In order to use these advantages to full capacity, the material flow into the area of the geometry details needs to be optimised. The key element for the material flow control is a multi-point blank holder. In combination with flange draw-in sensors, a closed loop flange draw-in control can be built up which guarantees a reproducible material flow and, consequently, defined part properties. Furthermore, a favourable pre-distribution of sheet metal material can be reached which leads to a widening of the process limits. Considering a large area sheet metal part with a complex door handle element as example, strategies for the material flow control will be discussed in this paper. The conclusions are based on FE-simulations as well as experimental findings.Article Citation - WoS: 38Citation - Scopus: 42Application of Continuum Damage Mechanics in Discontinuous Crack Formation: Forward Extrusion Chevron(Wiley-v C H verlag Gmbh, 2008) Soyarslan, Celal; Tekkaya, A. Erman; Akyuz, Ugurhan; Department of Mechatronics Engineering; Manufacturing EngineeringMaterializing Continuum Damage Mechanics (CDM), numerical modeling of discrete internal cracks, namely central bursts, in direct forward extrusion process is presented. Accordingly, in a thermodynamically consistent setting, a local Lemaitre variant damage model with quasi-unilateral evolution is coupled with hyperelastic-plasticity. The formulations are constructed in the principal axes where simultaneous local integration schemes are efficiently developed. To this end, the framework is implemented as ABAQUS/VUMAT subroutine to be used in an explicit FE solution scheme, and utilized in direct forward extrusion simulations for bearing steel, 100Cr6. Discontinuous cracks are obtained with the element deletion procedure, where the elements reaching the critical damage value are removed from the mesh. The periodicity of the cracks shows well accordance with the experimental facts. The investigations reveal that, application of the quasi-unilateral conditions together with the crack closure parameter has an indispensable effect on the damage accumulation zones by determining their internal or superficial character. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Article Citation - WoS: 60Citation - Scopus: 68A Semi-Empirical Approach for Residual Stresses in Electric Discharge Machining (edm)(Elsevier Sci Ltd, 2006) Ekmekci, Bulent; Tekkaya, A. Erman; Erden, Abdulkadir; Department of Mechatronics Engineering; Manufacturing EngineeringHigh residual stresses are developed on the surfaces of electric discharge machined parts. In this study, layer removal method is used to measure the residual stress profile as a function of depth beneath the surface caused by die sinking type EDM. Cracking and its consequences on residual stresses are also studied on samples machined at long pulse durations. A modified empirical equation is developed for scaling residual stresses in machined surfaces with respect to operating conditions. In this model, a unit amplitude shape function representing change in curvature with respect to removal depth is proposed. The proposed form is found to be a special form of a Gauss Distribution. It is the sum of two Gaussian peaks, with the same amplitude and pulse width but opposite center location. The form can be represented by three constant coefficients. These coefficients depend on the released energy by a power function. (C) 2005 Elsevier Ltd. All rights reserved.Article Citation - WoS: 15Finite deformation plasticity coupled with isotropic damage: Formulation in principal axes and applications(Elsevier Science Bv, 2010) Soyarslan, C.; Tekkaya, A. E.; Manufacturing Engineering; Department of Mechatronics EngineeringA local, isotropic damage coupled hyperelastic-plastic framework is formulated in principal axes. It is shown that, in a functional setting, treatment of many damage growth models, including those originated from phenomenological models (with formal thermodynamical derivations), micromechanics or fracture criteria, proposed in the literature, is possible. As a model problem, a Lemaitre-variant damage model with quasi-unilateral damage evolutionary forms is given with special emphasis on the feasibility of formulations in principal axes. To this end, closed form expression for the inelastic tangent moduli, consistent with the linearization of the closest point projection algorithm, is derived. It is shown that, generally, even in the absence of quasi-unilateral damage evolutionary conditions, the consistent tangent moduli are unsymmetric. The model is implemented as a user defined material subroutine (UMAT) for ABAQUS/Standard. The predictive capability of the selected model problem is studied through axi-symmetric application problems involving forward extrusion of a cylindrical billet, upsetting of a tapered specimen and tension of a notched specimen, in which characteristic failure mechanisms are observed. (C) 2010 Elsevier B.V. All rights reserved.Conference Object Citation - WoS: 11Modeling Flexforming (fluid Cell Forming) Process With Finite Element Method(Trans Tech Publications Ltd, 2007) Hatipoglu, H. Ali; Polat, Naki; Koeksal, Arif; Tekkaya, A. Erman; Manufacturing EngineeringIn this paper, the flexforming process is modeled by finite element method in order to investigate the operation window of the problem. Various models are established using explicit approach for the forming operation and implicit approach for the unloading one. In all analyses the rubber diaphragm has been modeled revealing that the modeling of this diaphragm is essential. Usine the material Aluminum 2024 T3 alclad sheet alloy, three basic experiments are conducted: Bending of a straight flange specimen, bending of a contoured flange specimen and bulging of a circular specimen. By these experiments tile effects of blank thickness, die bend radius, flange length and orientation of the rolling direction of the part have been investigated. Experimental results are compared with finite element results to verify the computational models.Article Citation - WoS: 25Citation - Scopus: 35A Guide for Validation of Fe-Simulations in Bulk Metal Forming(Springer Heidelberg, 2005) Tekkaya, AE; Manufacturing Engineering; Manufacturing EngineeringNumerical analysis of metal forming processes is an everyday practice in industry. Forming loads, material flow, forming defects such as underfills, laps, and even cracks, stresses in dies and punches, as well as product properties like new hardness distribution, dimensional accuracies, and residual stresses are predicted by numerical analysis and used for technology generation. Most of the numerical analysis is done by the finite element method made available for engineers and technicians by numerous powerful commercial software packages. These software packages act as black-boxes and usually hide the complicated numerical procedures and even their crucial parameters from the applier. Therefore, the question arises during industrial applications: how accurate is the simulation, and how can the results be assessed? The aim of this paper is to provide a guideline to assess the results of metal forming simulations. Although some ideas are valid for any metal forming process, bulk forming is the primary concern. The paper will address firstly the possible sources of error in a finite element analysis of bulk forming processes. Then, some useful elementary knowledge will be summarized. Various levels of validation such as result and ability validation and assessment will be discussed. Finally, interpretation of results will be treated. In this content also some suggestions will be given.Conference Object Citation - Scopus: 2Influence of Heat Treatment and Grinding Conditions on Surface Residual Stresses in the Production of Rollers(Trans Tech Publications Ltd, 2006) Güley,V.; Tekkaya,A.E.; Savaş,T.; Özhan,F.; Manufacturing EngineeringThe aim of this study is to investigate surface residual stresses after heat treatment and grinding processes in the production of rollers. The residual stresses were measured using the X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 μm incident on AISI-E52100 (100Cr6) ball bearing steel. Taguchi design of experiments (DOE) is applied to define the set of experiments for grinding, which facilitates evaluation of the individual influences of process parameters on residual stresses and also eliminates unnecessary experiments. Response of residual stresses to each parameter is evaluated with the help of the results of residual stress measurements by X-ray diffraction. In grinding with aluminum oxide wheels, it was concluded that the lower the cutting speed and the higher the workpiece speed the higher the magnitude of surface compressive residual stresses. Higher compressive stresses were measured in axial direction compared to the circumferential direction after the grinding process.
